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  Java-基础-死锁
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      <h1 id="Java-基础-死锁"><a href="#Java-基础-死锁" class="headerlink" title="Java-基础-死锁"></a>Java-基础-死锁</h1><h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><ul>
<li><strong>所谓死锁，是指多个进程在运行过程中因争夺资源而造成的一种僵局</strong>，当进程处于这种僵持状态时，若无外力作用，它们都将无法再向前推进。 因此我们举个例子来描述，如果此时有一个线程A，按照先锁a再获得锁b的的顺序获得锁，而在此同时又有另外一个线程B，按照先锁b再锁a的顺序获得锁</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200817-161342794.png" alt="mark"></p>
<a id="more"></a>



<h2 id="1-产生死锁的原因"><a href="#1-产生死锁的原因" class="headerlink" title="1. 产生死锁的原因"></a>1. 产生死锁的原因</h2><p>可归结为如下两点：</p>
<p><strong>a. 竞争资源</strong></p>
<ul>
<li>系统中的资源可以分为两类：</li>
</ul>
<ol>
<li>可剥夺资源，是指某进程在获得这类资源后，该资源可以再被其他进程或系统剥夺，CPU和主存均属于可剥夺性资源；</li>
<li>另一类资源是不可剥夺资源，当系统把这类资源分配给某进程后，再不能强行收回，只能在进程用完后自行释放，如磁带机、打印机等。</li>
</ol>
<ul>
<li>产生死锁中的竞争资源之一指的是竞争不可剥夺资源（例如：系统中只有一台打印机，可供进程P1使用，假定P1已占用了打印机，若P2继续要求打印机打印将阻塞）</li>
<li>产生死锁中的竞争资源另外一种资源指的是竞争临时资源（临时资源包括硬件中断、信号、消息、缓冲区内的消息等），通常消息通信顺序进行不当，则会产生死锁</li>
</ul>
<p><strong>b. 进程间推进顺序非法</strong></p>
<ul>
<li>若P1保持了资源R1,P2保持了资源R2，系统处于不安全状态，因为这两个进程再向前推进，便可能发生死锁</li>
<li>例如，当P1运行到P1：Request（R2）时，将因R2已被P2占用而阻塞；当P2运行到P2：Request（R1）时，也将因R1已被P1占用而阻塞，于是发生进程死锁</li>
</ul>
<h2 id="2-四个必要条件"><a href="#2-四个必要条件" class="headerlink" title="2. 四个必要条件"></a>2. 四个必要条件</h2><p>产生死锁的必要条件：</p>
<ol>
<li><strong>互斥条件</strong>：进程要求对所分配的资源进行排它性控制，即在一段时间内某资源仅为一进程所占用。</li>
<li><strong>请求和保持条件：</strong>当进程因请求资源而阻塞时，对已获得的资源保持不放。</li>
<li><strong>不剥夺条件：</strong>进程已获得的资源在未使用完之前，不能剥夺，只能在使用完时由自己释放。</li>
<li><strong>环路等待条件：</strong>在发生死锁时，必然存在一个进程–资源的环形链。</li>
</ol>
<h2 id="3-预防死锁"><a href="#3-预防死锁" class="headerlink" title="3. 预防死锁"></a>3. 预防死锁</h2><ul>
<li><strong>资源一次性分配</strong>：一次性分配所有资源，这样就不会再有请求了：（破坏请求条件）</li>
<li>只要有一个资源得不到分配，也不给这个进程分配其他的资源：（破坏请保持条件）</li>
<li>可剥夺资源：即当某进程获得了部分资源，但得不到其它资源，则释放已占有的资源（破坏不可剥夺条件）</li>
<li><strong>资源有序分配法</strong>：系统给每类资源赋予一个编号，每一个进程按编号递增的顺序请求资源，释放则相反（破坏环路等待条件）</li>
</ul>
<p><strong>1、以确定的顺序获得锁</strong></p>
<p>如果必须获取多个锁，那么在设计的时候需要充分考虑不同线程之前获得锁的顺序。按照上面的例子，两个线程获得锁的时序图如下</p>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200817-170806789.png" alt="mark"></p>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200817-170822430.png" alt="mark"></p>
<ul>
<li>那么死锁就永远不会发生。 针对两个特定的锁，开发者可以尝试<strong>按照锁对象的hashCode值大小的顺序，分别获得两个锁，这样锁总是会以特定的顺序获得锁，那么死锁也不会发生</strong></li>
<li>问题变得更加复杂一些，如果此时有多个线程，都在竞争不同的锁，简单按照锁对象的hashCode进行排序（<strong>单纯按照hashCode顺序排序会出现“环路等待”），可能就无法满足要求了，这个时候开发者可以使用银行家算法</strong>，所有的锁都按照特定的顺序获取，同样可以防止死锁的发生，该算法在这里就不再赘述了，有兴趣的可以自行了解一下。</li>
</ul>
<p><strong>2、超时放弃</strong></p>
<ul>
<li>当使用synchronized关键词提供的内置锁时，只要线程没有获得锁，那么就会永远等待下去，然而Lock接口提供了<code>boolean tryLock(long time, TimeUnit unit) throws InterruptedException</code>方法，该方法可以按照固定时长等待锁，</li>
<li>因此线程可以在获取锁超时以后，主动释放之前已经获得的所有的锁。通过这种方式，也可以很有效地避免死锁。 还是按照之前的例子，时序图如下：</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200817-171034944.png" alt="mark"></p>
<h2 id="4-避免死锁"><a href="#4-避免死锁" class="headerlink" title="4. 避免死锁"></a>4. 避免死锁</h2><ul>
<li>预防死锁的几种策略，会严重地损害系统性能。因此在避免死锁时，要施加较弱的限制，从而获得 较满意的系统性能。由于在避免死锁的策略中，允许进程动态地申请资源。因而，系统在进行资源分配之前预先计算资源分配的安全性。若此次分配不会导致系统进入不安全的状态，则将资源分配给进程；否则，进程等待。其中<strong>最具有代表性的避免死锁算法是银行家算法</strong>。</li>
<li><strong>银行家算法</strong>：<ul>
<li>首先需要定义状态和安全状态的概念。系统的状态是当前给进程分配的资源情况。</li>
<li>因此，状态包含两个向量Resource（系统中每种资源的总量）和Available（未分配给进程的每种资源的总量）及两个矩阵Claim（表示进程对资源的需求）和Allocation（表示当前分配给进程的资源）。</li>
<li>安全状态是指至少有一个资源分配序列不会导致死锁。当进程请求一组资源时，假设同意该请求，从而改变了系统的状态，然后确定其结果是否还处于安全状态。如果是，同意这个请求；如果不是，阻塞该进程知道同意该请求后系统状态仍然是安全的。</li>
</ul>
</li>
</ul>
<h2 id="5-检测死锁"><a href="#5-检测死锁" class="headerlink" title="5. 检测死锁"></a>5. 检测死锁</h2><ol>
<li>首先为每个进程和每个指定资源指定一个唯一的号码</li>
<li>然后建立资源分配表和进程等待表。</li>
</ol>
<h2 id="6-解除死锁"><a href="#6-解除死锁" class="headerlink" title="6. 解除死锁"></a>6. 解除死锁</h2><p>当发现有进程死锁后，便应立即把它从死锁状态中解脱出来，常采用的方法有：</p>
<ul>
<li>剥夺资源：从其它进程剥夺足够数量的资源给死锁进程，以解除死锁状态；</li>
<li>撤消进程：可以直接撤消死锁进程或撤消代价最小的进程，直至有足够的资源可用，死锁状态.消除为止；所谓代价是指优先级、运行代价、进程的重要性和价值等。</li>
</ul>
<h2 id="7-死锁检测"><a href="#7-死锁检测" class="headerlink" title="7. 死锁检测"></a>7. 死锁检测</h2><p><strong>1、Jstack命令</strong></p>
<ul>
<li>jstack是java虚拟机自带的一种<strong>堆栈跟踪工具</strong>。jstack用于打印出给定的java进程ID或core file或远程调试服务的Java堆栈信息。 Jstack工具可以用于生成java虚拟机当前时刻的线程快照。线程快照是当前java虚拟机内每一条线程正在执行的方法堆栈的集合，生成线程快照的主要目的是定位线程出现长时间停顿的原因，如线程间死锁、死循环、请求外部资源导致的长时间等待等。 线程出现停顿的时候通过jstack来查看各个线程的调用堆栈，就可以知道没有响应的线程到底在后台做什么事情，或者等待什么资源。</li>
</ul>
<p><strong>2、JConsole工具</strong></p>
<ul>
<li>Jconsole是JDK自带的监控工具，在JDK/bin目录下可以找到。它用于连接正在运行的本地或者远程的JVM，对运行在Java应用程序的资源消耗和性能进行监控，并画出大量的图表，提供强大的可视化界面。而且本身占用的服务器内存很小，甚至可以说几乎不消耗。 </li>
</ul>
<p><strong>参考博客： <a href="https://blog.csdn.net/hd12370/article/details/82814348" target="_blank" rel="noopener">https://blog.csdn.net/hd12370/article/details/82814348</a></strong></p>

      
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